Development of a micro-combined heat and power powered by an opposed-piston engine in building applications

Residential homes and light commercial buildings usually require substantial heat and electricity simultaneously. A combined heat and power system enables more efficient and environmentally friendly energy usage than that achieved when heat and electricity are produced in separate processes. However, due to financial and space constraints, residential and light commercial buildings often limit the use of traditional large-scale industrial equipment. Here we develop a micro–combined heat and power system powered by an opposed-piston engine to simultaneously generate electricity and provide heat to residential homes or light commercial buildings. The developed prototype attains the maximum AC electrical efficiency of 35.2%. The electrical efficiency breaks the typical upper boundary of 30% for micro–combined heat and power systems using small internal combustion engines (i.e., <10 kW). Moreover, the developed prototype enables maximum combined electrical and thermal efficiencies greater than 93%. The prototype is optimally designed for natural gas but can also run renewable biogas and hydrogen, supporting the transition from current conventional fossil fuels to zero carbon emissions in the future. The analysis of the unit’s decarbonization and cost-saving potential indicate that, except for specific locations, the developed prototype might excel in achieving decarbonization and cost savings primarily in US northern and middle climate zones.


Supplemental Note 1: Summary of mCHP technologies available
The major micro-combined heat and power (mCHP) technologies include internal combustion engines (ICEs), micro gas turbines, micro-Rankine cycles, Stirling engines, and thermophotovoltaic generators [3,[18][19][20][21][22][23][24][25][26][27][28].Table S1 summarizes these representative technologies available in the public domain.Clearly, their performance is substantially different.This considerable variation in performance is a direct result of employing different mCHP technologies, design and fabrication approaches, and system integration and optimization considerations, all while considering cost-effectiveness.Additionally, specific procedures and standards in different countries may also contribute to these differences.Note: CHP = combined heat and power; ORC = organic Rankine cycle; TPV = thermophotovoltaic.

Supplemental Note 2: Ten testing cases for evaluating the mCHP
Six cases were tested under stoichiometric combustion modes with the air-fuel ratio set to λ = ~1.0.These cases are listed in Table S2.The first three cases measured the electrical efficiency and overall mCHP thermal efficiency with waste heat recovered and stored in the water tank at various electric power generation ratings.The fourth, fifth, and sixth cases measured the electrical efficiency and overall mCHP thermal efficiency with external thermal demand hot water supply and space heating at different electric power generation capacities.Table S2.Six testing cases for evaluating the mCHP under stoichiometric combustion modes Case Description 1 3.5 kW electricity with waste heat recovered and stored in the water tank; the water tank is heated from 26.7 °C to 60 °C without external heat demand 2 4.5 kW electricity with waste heat recovered and stored in the water tank; the water tank is heated from 26.7 °C to 60 °C without external heat demand 3 7.5 kW electricity with waste heat recovered and stored in the water tank; the water tank is heated from 61.1 °C to 71.1 °C without external heat demand 4 3.5 kW electricity with external thermal demand for space heating; the water tank temperature starts at 40 °C 5 4.5 kW electricity with external thermal demand for space heating; the water tank temperature starts at 40 °C 6 6.0 kW electricity with external thermal demand for space heating; the water tank temperature starts at 40 °C Four cases were tested under lean combustion modes, shown in Table S3.The fourth case aims to repeat the third case and was used to calculate the uncertainty associated with the measurements.
All the lean cases recovered waste heat and stored it in the tank.In all the lean combustion cases, the control module used the lambda sensor and intake port actuator to maintain a 30% excess of air consumption or λ = ~1.3, and the ignition timing for all lean modes was advanced approximately 10 °CA to maximize torque operation compared with the stoichiometric modes.

Supplemental Note 3: Measurement uncertainties
The propagation of the uncertainty method [18] was used to calculate the uncertainty associated with the measurements in the work.The method estimates the uncertainty in a parameter from the uncertainties in the measurements used to calculate the parameter.For example, the uncertainty of energy in the burnt fuel depends on the uncertainties of fuel flow measurement and lower heating value (LHV) calculation in the testing cases.In the same manner, the uncertainty of total efficiency of the system is a function of measurement uncertainties in temperature, pressure, and mass flow rates.The key instruments involved in the measurements for the calculation of mCHP efficiencies are listed in Table S4.In the current system, the uncertainties of electrical, thermal, and total efficiency are derived as 3.28%, 2.26%, and 2.51%, respectively.All the experimental uncertainty has been shown in Figure 4 in the revised manuscript.

Supplemental Note 4: The sensitivity comparison of the repeated tests
Regarding test repeatability and reproducibility, the authors repeated the test for the 5.93 AC kW of the lean combustion modes (i.e., the cases 9 and 10).The repeatability results are shown in Table S5.The results from Figure S1 reveal that the maximum sensitivity is less than 1.8% for all the performing efficiencies; power; opposed-piston four-stroke (OP4S) exhaust; and coolant.It indicates that the mCHP is capable of excellent repeatability and reproducibility.

Supplemental Note 6: The electrical Performance of the mCHP system
For the stoichiometric combustion modes, Table S7 shows the electrical power outputs of the mCHP with peak AC electricity efficiency of 26.4%.The power outputs are in the range of 3.33-7.70AC kW (i.e., 3.20-7.39DC kW) for the six selected cases.The AC electricity efficiencies vary between 16.8% and 26.4%, and the DC electricity efficiencies vary between 16.1% and 25.3%.
The results reveal that larger electric power output leads to higher electrical efficiency, whether waste heat is recovered and stored in the water tank or external thermal load is applied for hot water supply and space heating.This expected result occurs because engine throttling loss is usually reduced with higher engine power.

Natural gas engine OP4S (Lean)
Supplemental Note 9: The mCHP performance of case 1 Figure S3 shows an example of the mCHP prototype under testing conditions of case 1 (i.e., 3.89 AC kW or 3.74 DC kW), in which waste heat was recovered and stored in the water tank.In this case, the water tank was heated from 26.7 °C to 60 °C.The water temperature at the upper location was heated, but the bottom location remained at 26.6 °C.Significant temperature stratification occurred in the water tank.This result was confirmed by performing high-fidelity computational fluid dynamics simulations via Ansys Fluent commercial software.The simulation results for the waste heat recovery component are shown in Supplemental Note 10.The emissions, including CO, HC, and NOx, were also measured from all the lean and stoichiometric modes.The results are shown in Figure S8, which reveals that the prototype using natural gas meets US EPA new source performance standards (NSPSs) for emissions for sparkignition stationary engines used in the power generation of less than 19kW.For engine displacement of the mCHP, the NSPS emission standards require CO emissions of no more than 610 g/kWh and HC+NOx emissions of no more than 8 g/kWh.Supplemental Note 15: Exergy analysis

• Methodology of Exergy Transfer and Destruction
In the exergy analysis, effectiveness of the mCHP system is evaluated based on the maximum theoretical work defined as exergy relative to the standard environment state.The temperature and pressure of the reference environment were set as  0 = 298 K and  0 = 1 atm, and the exergy reference environmental air was assumed to be 75.67%N2, 20.35% O2, 3.12% H2O, 0.03% CO2, and 0.83% other species [20,21].
By assuming a steady-state condition ignoring kinetic and potential energy, the exergy rate equation for each component in the mCHP (i.e., engine, generator, waste heat recovery component, and other components) can be calculated with the following equation: where  ̇ is the heat transfer rate at the boundary of a given component;   is the temperature at the boundary where heat transfer occurs;  0 is the reference temperature;  ̇ is the rate of work delivered including mechanical work and electrical work;  ̇ and  ̇ are the rates of exergy transfer into and out of a given component, respectively, owing to mass transfer into and out of the component; and  ̇ is the destructed exergy during the irreversible process.
For the engine component, the analysis assumed that the intake air was introduced at the same condition as that of the reference environment.Therefore, the intake air exergy was ignored in the study.The fuel exergy, ̅  ℎ , was calculated based on the equation below: where ̇  is the fuel model rate, and   and ̅  ℎ represent the mole fraction of each species and the standard chemical exergy of each species.For natural gas, three main components-CH4, C2H6, and C3H8-are considered and listed in Table S11.The exergy of the engine exhaust flow is given as the sum of thermomechanical exergy, ̅ ℎ , and chemical exergy, ̅ ℎ .
The specific thermomechanical exergy of the engine exhaust flow is defined as Therefore, the specific thermomechanical exergy can be estimated using the following equation: where ̇ ℎ is the exhaust mole flow rate; ℎ ̅ ℎ and ̅ ℎ are the specific enthalpy and entropy of the engine exhaust at a state, respectively; and subscript 0 denotes the reference state at  0 and  0 .
In addition, ̅ ℎ 0 is the absolute entropy of the exhaust flow,  ̅ is the universal gas constant, and  ℎ is the pressure of the engine exhaust.
The chemical exergy of the engine exhaust flow can be estimated by The exergy of the coolant flow is given as )), and the exergy of the engine heat loss is given as where ̇  and   are the mass flow rate and specific heat of the engine coolant, respectively;   is the coolant temperature;  ̇,ℎ is the engine heat loss;   is the engine surface temperature; and  ̇,ℎ is estimated based on the remaining heat of fuel energy minus power, exhaust energy and coolant energy.
Thus, based on Eqs.(S1)-(S10), a thorough exergy analysis of the engine component was conducted to account for the exergy associated with fuel, work, exhaust gas, engine coolant, engine heat loss, and mechanical work.The exergy destroyed during the irreversible combustion process is determined by contrasting the exergy of the fuel with the residual exergy mentioned earlier in this section.In addition, the condensation of water in exhaust gas exiting the waste heat recovery system was considered based on the saturation pressure at the exhaust gas temperature exiting the waste heat recovery system.
For the waste heat recovery at the water tank, the exergy transfer from the exhaust gas and coolant to the water stored in the water tank was considered.The exergies available in the exhaust gas and coolant were used as inputs for the waste heat recovery component.To simplify the analysis, it was assumed that there was no additional heat loss between the engine and the waste heat recovery component.The exergy recovered from exhaust gas and coolant to the water tank was estimated based on the energy and exergy balances among exhaust gas, coolant, and tank water, shown in Eqs.(S11) and (S12).where ∆ ℎ is calculated based on the exhaust flow rate and temperature difference through the coil implemented in the water tank.A similar method is used to calculate ∆  based on coolant flow and their temperatures at the inlet and exit of the coil implemented in the water tank.Water tank heat loss, ∆ , , is small owing to its excellent insulation (see Figure 1) and is assumed to be 0.1 kW.In addition, ∆ ̇ℎ and ∆ ̇ are the exergy variations through the exhaust and coolant coils, respectively, in the water tank;  ̇ℎ .and  ̇ℎ, are the recovered exergy and exergy loss in the water tank, respectively; and ∆ ̇ℎ , ∆ ̇ and  ̇ℎ are analyzed based on Eq.
For electrical components such as a generator and a rectifier, most of the terms in Eq. (S1) can be ignored, except for the  ̇ and  ̇.Electricity generation was considered as  ̇.The loss at the electrical components was given as • Discussion of the entire CHP system analysis In the CHP system analysis, fuel exergy shall be equal to the sum of electric generation, heat loss, exhaust loss, and destruction exergy at each component.In Eq. (S14),  ̇,&ℎ is exergy loss due to irreversible combustion and heat loss from the combustion chamber to coolant and oil.
̇ =  ̇ + ∑ ̇ℎ + ∑ ̇ℎ  + ∑ ̇,&ℎ (S14)     Supplemental Note 16: Fuel types of the data shown in Figure 6 Table S14 summarizes fuel types of the data shown in Figure 6.The majority of fuels used in the data are natural gas.This is expected owing to natural gas's complete pipeline infrastructure and mature market.

Supplemental Note 17: Ten representative homes covering northern, middle, and southern climate zones
In the United States in 2023, electricity was produced from wide ranging sources such as coal (14.9%), hydroelectric (6.2%), natural gas (40.1%), nuclear power (19.6%), solar (4.4%), wind energy (13.5%), and others (2.3%) [22].Fossil fuels such as coal and natural gas are still predominant, but renewable sources such as wind and solar are growing quickly in several states.
For example, Texas led all US states in renewable energy production, accounting for over 23% of the nation's totals.Consequently, carbon intensity of electricity generation (i.e., kilograms of CO2 per kilowatt-hour of power generation), as well as electricity retail price, vary substantially at different locations.To reasonably evaluate the benefits and disadvantages of the mCHP prototype performance in the decarbonization and operation cost savings of single household applications, 10 homes were selected, respectively, from cities in 10 states representing northern, middle, and southern climate zones in the United States.Figure S11 shows the locations of these representative homes as well as their different CO2 kilograms per kilowatt-hour of power generation and diverse retail prices for electricity and natural gas.Figure S11 also shows the data for kilograms of CO2 per kilowatt-hour of power generation, electricity retail prices, and natural gas retail prices, which are adopted from US Energy Information Administration data [23,24,25].less natural gas for space heating between November and April, and no natural gas was used in Fort Worth.Additionally, the 10 homes require near-zero natural gas consumption in summertime (i.e., July to August).By contrast, annual electricity consumption of the homes in the southern climate zone is higher than that of the homes in the northern climate zone.The information is useful for effectively understanding the potential benefits and disadvantages of the mCHP prototype in US household applications.Northern Climate Zone Southern Climate Zone Middle Climate Zone residential electricity price;  , ,  ℎ, , and  ℎ, are the present maintenance cost for furnace, water heater, and mCHP, respectively;  , ,  ℎ, ,  , , and  ℎ, are the present initial costs of the furnace and water heater, as well as their installation costs;  ℎ, ,  , ,  ℎ, , and  , are the present initial costs of mCHP and battery systems, as well as their installation costs;  , ,  ℎ, ,  ℎ, , and  , are the current disposal costs for the furnace, water heater, mCHP, and battery;  , ,  ,ℎ ,  ,ℎ , and  , are the disposal costs of a furnace, water heater, mCHP, and battery at their life span (including their replacement disposal); and   ,  ℎ , and   are the life span of a residential furnace, water heater, and battery system.All the present initial, maintenance, and disposal costs of these device is listed in Table S15.In Eq.S16,  ̇ℎ_ () and  ̇ℎ_ () are assessed based on the selected home data, and  ̇ℎ  () and  ̇ℎ_ () are determined by an operating strategy designed to maximally utilize the electricity and waste heat from the mCHP for a single household application.In that strategy, the mCHP operates optimally by switching between stoichiometric and lean modes, considering a trade-off of cost savings and carbon emissions reduction while meeting thermal energy and electricity demands in a home.In each mCHP operation mode, the electricity output is used to satisfy household power demand.However, if the mCHP electricity output falls short of meeting household power demand, grid electricity buffers the excess power demand.Similarly, the mCHP waste heat is used to satisfy household thermal energy demand for space and water heating typically fulfilled by natural gas.However, if the mCHP waste heat is insufficient to satisfy thermal energy demand, the mCHP uses its electricity output, along with grid electricity, to buffer the additional thermal energy demand.This operating strategy aims to maximize the utilization of the mCHP energy output.
In the detailed strategy, the mCHP is assumed to run lean mode and stoichiometric modes (i.e., cases 5 and 9, respectively  In addition, to evaluate the environmental effect of mCHP, life cycle CO2 emission analysis was also performed based on mCHP's carbon emissions associated with gas consumption plus carbon emissions associated with electricity imported from the grid, minus CO2 generated in gas and electricity consumption in residential home devices, given as below: where ∆2 1 and ∆2  are CO2 emissions reduction over the period of 1 year and mCHP life cycle.The equation above assumes US grid transmission and distribution loss   = 4% [27]; US natural gas transmission and distribution loss   = 5% [28]; residential natural gas price   depends on the simulated locations [26], shown in Figure S11; residential electricity price   depends on the simulated locations [25], shown in Figure S11; CO2e per kilowatt electricity generation,  2  , depends on the simulated locations [24], also shown in Figure S9; and CO2e per kilowatt natural gas consumption  2  = 180.5 g/kWh [29].

Figure
Figure S2 compares engine brake thermal energy efficiency between the opposed-piston engineand Cummins Westport natural gas engine as a function of engine power outputs.The Cummins Westport natural gas engine is a 2018 6.7 L, four-cycle, spark-ignition, in-line six-cylinder engine[19].

5 Figure
FigureS3.The mCHP prototype performance under testing conditions of 3.89 AC kW (i.e., 3.74 DC kW) and with waste heat recovered and stored in the water tank.

mCHP performance of case 9 Figure
FigureS6shows the mCHP prototype performance under the testing conditions of 5.93 AC kW (i.e., 5.7 DC kW) under the lean combustion mode (Case 9) and with waste heat recovered and stored in the water tank.

Figure S6 .
FigureS6.The mCHP prototype performance under testing conditions of 5.93 AC kW (i.e., 5.7 DC kW) under the lean combustion mode and with waste heat recovered and stored in the water tank.

Figure S7 . 14 :
Figure S7.Comparison of latent heat loss due to uncondensed water vapor between lean and stoichiometric modes.In the embedded figure, the percentage of the uncondensed water latent heat loss is based on HHV fuel energy.Note: red circles are stoichiometric modes, and blue circles are lean modes.

Figure
Figure S8.(a) HC+NOx emissions of all the lean and stoichiometric modes as a function of engine power and (b) CO emissions of all the lean and stoichiometric modes as a function of engine power.The error bars of HC+NOx and CO emissions shown are 10%.Note: red circles are stoichiometric modes, and blue circles are lean modes.

Figure S12 shows 1 -
Figure S12 shows 1-year data of real-world electricity consumption and natural gas for space and water heating in the homes, downloaded from the EnergyPlus residential energy consumption database [26].This database covers 1 year of real-world hour-by-hour measurements from 237single houses across all 50 US states.FigureS13compares the annual household electricity and heating gas consumption of the 10 households and clearly reflects that these homes experience different load demand because of diverse climate conditions.In the northern climate zone, the four homes in Anchorage, Boston, Detroit, and Minneapolis require substantial natural gas for space heating between November and April, and they still must use natural gas for water heating even between May and October.However, the homes in the southern climate zone require substantially

Figure S12 .Figure S13 .
Figure S12.One year's data of real-world electricity and natural gas consumption for space and water heating of the 10 representative homes located in Anchorage, Alaska; Boston, Massachusetts; Detroit, Michigan; Minneapolis, Minnesota; Boulder, Colorado; Kansas City, Missouri; Lexington, Kentucky; Atlanta, Georgia; Fort Worth, Texas; and Los Angeles, California.

Figure S15 .
Figure S15.The (a) natural gas energy consumption and (b) waste heat loss of mCHP applications in the 10 households.

Table S1 .
Summary and characteristics of mCHPs for single-family houses and small buildings

Table S3 .
Four testing cases for evaluating the mCHP under lean combustion modes within the tank is heated from 61.1 °C to 71.1 °C without external heat demand.Additionally, the cases 4-6 are designed because typical US domestic space heating temperature, generated from heat pumps and residential furnaces, is in the range of 21.1 °C to 51.7 °C (70 °F to 125 °F).These cases have the water tank beginning at 40 °C and incorporating external thermal demand for space heating requirements.

Table S4 .
Measurement range and sensitivity of instruments used in the tests.

Table S5 .
Repeatability and reproducibility results for the mCHP.
Repeatability resolution analysis for cases 9 and 10.Note: the error bar shows the maximum sensitivity of mCHP test repeatability and reproducibility.

Table S6 .
Fuel composition of natural gas used in the testing cases shown in Figure4.

Table S8 .
Electrical efficiency performance of the mCHP prototype in lean combustion mode.

Supplemental Note 7: The Thermal Performance of the mCHP systemTable S9 .
Thermal energy performance of the mCHP prototype under the stoichiometric modes.

Table S10 .
Thermal energy performance of the mCHP prototype in the lean combustion mode.

Exhaust Flow Coolant Flow Total Thermal Efficiency Supplemental Note 13: Lean combustions modes cause higher latent heat loss
Two factors result in lower overall mCHP efficiencies in lean combustion modes.First, lean combustion modes lead to exhaust temperatures at the water tank exit ranging from 43.4 °C to 47.7 °C, indicating potential for improvement in the thermal energy control system for enhanced waste heat recovery.Second, lean combustion modes inherently lead to a lower ratio of moisture to dry air, increasing uncondensed water in the exhaust flow rejected to ambient conditions and causing higher latent heat loss.In the studies, the latent heat loss due to uncondensed water vapor in lean modes is 5.3%-~6.9% of HHV fuel energy.The loss in the stoichiometric modes is 2.2%-2.8% of HHV fuel energy with the exhaust temperature at the exit of water tank below 40 °C, but the loss is 9.2% of HHV fuel energy in the stoichiometric modes with the exhaust temperature at the exit of the water tank above 70 °C.The observation from FigureS7illustrates that there is no condensate in stoichiometric modes if the exhaust temperature at the exit of the water tank is above 60 °C.That means all latent heat is lost to ambient condition.

Table S11 .
Specific chemical exergy of main fuel components.
and    are mole fractions of the i th species in the exhaust mixture at a state and in the reference environment, and ̅  ℎ is the standard chemical exergy, which can be obtained from the standard exergy table.The exhaust gas composition in the equation above was calculated by assuming the complete combustion.Thus, the global reaction equations of stoichiometric and 30% lean combustions employed are presented as follows: where

Table S12
summarizes the exergy flow and destruction in all 10 cases operating under the conditions listed in Supplemental Notes 2, 6, and 7. Figure S9 exhibits the Sankey diagrams of exergy flow and destruction in each component.Table S13 summarizes the energy flow and component loss in all 10 cases.Figure S10 further shows the Sankey diagrams of energy flow and losses in each component in all 10 cases.

Table S12 .
Summary of exergy flow and destruction of the 10 cases.314

Table S13 .
Summary of energy flow and loss of the 10 cases.345

Table S15 :
Present initial, maintenance, and disposal costs of mCHP, battery, furnace and water heater.

Table S16 .
Detailed mCHP annual repair and maintenance costs Annual

repair and maintenance Component cost and labor cost
). Case 5 in the stoichiometric mode enables 4.74 AC kW (i.e., 4.55 DC kW) power output and generates 21.52 kW waste heat at 92.3% overall mCHP efficiency (based on the higher heating value) and 18.4% AC efficiency (based on the lower heating value).Case 9 in the stoichiometric mode enables 5.93 AC kW (i.e., 5.7 kW) power output and generates 11.77 kW waste heat at 86.4% overall mCHP efficiency (based on the higher heating value) and 35.2% AC efficiency (based on the lower heating value).Evidently, the stoichiometric mode provides lower power and high thermal energy, and the lean mode delivers high power and lower thermal energy for house usage.Based on the power and thermal energy demand from a given house, the mCHP adopts a targeted function (see Eq.